Sounds like your TV and/or HDMI converter is not passing the 2160p60 signal. You need to rest with a monitor that can accept UHD like a Video Assist or other SDI UHD monitor. Wasn’t sure which switcher you are using, as their is not a ATEM Television Studio 4K in the current line up, and the ATEM Studio Pro 4K can convert input/output resolution independently. The ATEM 1 M/E Production Studio 4K requires the inout and output resolutions to match. You need to be more accurate with your product name, as it can get confusing…
If you have the 1 M/E ATEM it has a HDMI program out also for a program monitor. Note, the second SDI Program out is a HD only output, (see diagram, HD out circled in red) and if you connected to that, it will only output HD. The switch a,so has a UHD program out next to it. The ATEM Studio Pro 4K has a single SDI Program out and 8 Aux outputs, which display the program out. This gives you a 9 x 10-bit HD/UHD switchable outputs, so you can select the program out to be either UHD or HD, and the resolution/frame rate, as it does a standards conversion on the fly.
Both switchers have HDMI outputs as well, and are primarily SDI switchers, so not sure why you are trying to use a SDI to HDMI converter on the program out? Normally in most broadcast situations, you have a SDI monitor in line on the program out, which is daisy chained out to the program feed…
If you have an older legacy ATEM, then you could have a hardware issue, and will need to replace the switcher with a newer model, as they are not economically repairable.
Cheers